Experimental design and methods. Participants will be randomized to one of three groups receiving clonidine 0.1 mg, clonidine 0.2 mg, or placebo orally under double-blind conditions. The study will consist of a single 5-6 hr experimental session in which there will be baseline measures, drug administration, and four script-guided imagery sets, each followed by a period of data collection. Three hours after dosing (when peak plasma clonidine concentrations are reached), participants will be exposed to four scripts: one stress-inducing, one describing drug cues, and two with neutral content. The standardized script-guided imagery procedure has previously been shown to reliably induce negative affective states (stress scripts) and/or craving (stress and drug-cue scripts) compared to the neutral scripts and to have internal and external validity (Tiffany and Drobes, 1990; Maude-Griffin and Tiffany, 1996; Taylor et al., 2000; Sinha et al., 1999, 2000, 2003; Singleton et al., 2003; Tiffany and Haekeneworth, 1991; Elash et al., 1995; Drobes and Tiffany, 1997; Taylor et al., 2000; Singleton et al., 2003). Outcome measures will include subjective ratings of drug craving and mood, autonomic response (galvanic skin response [GSR]), and endocrine responses (salivary cortisol and salivary ?-amylase, a measure of endogenous adrenergic activity during stress (Chatterton et al., 1996; Nater et al., 2005; van Stegeren et al., 2005)).